The advent of anti-lock braking systems and traction control systems have created a need for accurate speed measurement of individual wheels of a vehicle. Accordingly, there has been an increase in motor vehicles equipped with speed sensing devices to measure axle rotation which, in turn, enables measurement of wheel speed.
Typically, a ferrous or magnetic exciter ring is installed to rotate with an axle. A sensor, fixed with respect to axle rotation, is placed in the vicinity of the teeth of the exciter ring. When an exciter ring is rotated near a variable reluctance sensor, the teeth on the exciter ring pass through the magnetic lines of flux generated by the magnet in the sensor. As the teeth are passing through the magnetic lines of flux, a voltage is generated in the coil within the sensor. The magnitude of the voltage is related to the speed and size of the exciter ring teeth in addition to design parameters inside the sensor, and inversely related to the distance between the sensor and the exciter ring teeth. The exciter ring assembly will also work similarly with an active (Hall Effect) sensor. The sensor is coupled to a control system which calculates the angular speed of the axle. The best sensor design selection is dependent upon the needs of the controller system. Calculation is carried out with inputs of the number of teeth sensed per unit of time and the known number of teeth of the exciter ring. During acceleration or deceleration, the instantaneous speed of the axle is not directly measurable by these speed sensing devices due to the finite number of teeth on the exciter ring. Thus, the accuracy of these devices is limited by the number of teeth of the exciter ring. A larger diameter exciter ring that provides more teeth can be used to obtain a more accurate speed measurement.
A variety of speed sensing devices have been used in the art. An example of a known speed sensing device is shown in U.S. Pat. No. 5,067,350 to Grillo et al. which discloses an annular exciter ring. Another example is disclosed in U.S. Pat. No. 5,967,669 to Ouchi. Here, a roller bearing unit includes an integral exciter ring.
It is known within the art to mount an exciter ring or target in the differential area of a driven axle. This location can usually accommodate a larger wheel since the differential housing within an axle assembly is typically of a larger diameter than the axle tube. One limitation to a speed sensing device located near the differential is the inaccuracy of the speed measurement associated with the elastic twisting of an axle under a torque loading or torque windup. While an axle is twisting, the actual wheel speed is not measured.
Alternatively, it is known to provide a speed sensing device that is integral with the wheel bearing assembly of an axle. This location reduces the inaccuracy due to torque windup, and increase complexity and mass. Additionally, the bearing assembly will have a higher cost and a different seal assembly may be required. While the above speed sensing devices may perform adequately for their intended purposes, a need exists for an improved exciter ring assembly that is less complex, provides for easier installation, and results in a more accurate measurement of actual wheel speed.